Variable strength output drivers are used in a variety of electronic devices. For example, in some memory systems, a driver with an adjustable drive strength may be used to match the capacitance of the address lines being driven, or to adjust the response times of the driven signals. Integrated circuits (ICs) may also contain variable strength output drivers to provide more flexibility in their target application. For example, a single general purpose IC may utilize a variable strength output driver to allow it to be used on a circuit board having a low parasitic load, or in a system having requiring a high drive strength. By adjusting the drive strength of a particular output driver to match the requirements of the target application, power consumption may be optimized and RF emissions caused by unnecessarily fast edge transitions may be reduced.
Variable strength output drivers may be implemented using parallel output drivers that are selectively activated according to a desired drive strength. These parallel output drivers may have different drive strengths, different gate lengths and different gate widths for each parallel output drive stage. In many cases, these variable strength output drivers are included in input/output (I/O) cells disposed near the periphery of the integrated circuit.
Such variable strength output drivers pose some difficulties with respect to sensitivity to electrostatic discharge (ESD). In the event of an ESD pulse at an I/O pin coupled to an output of the variable strength driver, selected parallel output device elements may conduct high ESD currents, while deselected elements may maintain a high impedance, thereby causing only a subset of the parallel output devices to handle large ESD currents. One way in which this issue has been addressed is by attaching ESD protection devices to the I/O pin coupled to the output of the variable strength driver.